L-TECTONITES FROM LINGTSE ORTHOGNEISSES IN THE SIKKIM HIMALAYAN FOLD-THRUST BELT: PRELIMINARY OBSERVATIONS

Paleoproterozoic felsic orthogneisses are an integral component of the Himalayan fold-thrust belt (FTB). Their contact with the lower Lesser Himalayan sequence (LHS) varies from gradational to thrust to intrusive as well as extrusive along different parts of the mountain belt. In the Darjeeling-Sikkim Himalaya, the equivalent Lingtse orthogneiss consistently shows a thrust contact with the underlying lower LHS. Here, structurally higher MCT1 places granulite facies Kanchenjunga-Darjeeling gneisses over amphibolite facies Paro -Lingste gneisses of the MCT2 sheet; MCT2 has the lower LHS in its footwall. The MCT sheets are folded by underlying Lesser Himalayan duplex (LHD) that has two structural components. Mylonitized Lingtse orthogneiss forms the MCT2 fault zone along parts of Sikkim Himalaya. These are also repeated as narrow, linear, N-S trending imbricates within the MCT2 sheet with a greater frequency in eastern Sikkim than in west. The absence of Lower LHD in eastern Sikkim, together with higher imbricates of Lingtse gneiss suggest lateral variation in partitioning of total shortening, and the importance of Lingtse imbricates in structural evolution of Sikkim Himalayan FTB.

We report L-tectonites from a linear, discontinuous klippe of Lingtse mylonite lying ~2 km west of the main MCT2 trace in eastern Sikkim. The klippe extends ~20 km in N-S, and ~3 km in E-W; the footwall rocks are lower LHS. In the northern part of the klippe, near Phodong, the quartz-feldspar-biotite-muscovite mylonitic gneiss forms a gently plunging (8°,006°), open, antiform; the deformation fabric varies from S>L (~46 m), L>S (~192 m), L(~176 m), S>L (~53 m), from west to east. The L tectonite zone coincides with the hinge zone, and has an average plunge of 12˚, 354˚. The surrounding lower LHS phyllites are S>>L tectonites. Stretched biotite grains, feldspar porphyroclasts, dynamically recrystallized quartz develop a pervasive, shape-preferred alignment in lineation parallel sections; this fabric is absent in lineation perpendicular sections. Dislocation creep in quartz, and microfracturing and dislocation glide in feldspar are the dominant deformation mechanisms in this zone. Preliminary results suggest rheologically driven strain path partitioning may have localized the L-tectonites in the homogeneous Lingtse orthogneiss.